In recent years, with advancement of digital technologies of electronic hardware, memory devices which have a large capacity and are nonvolatile have been developed vigorously to store data of music, image, information and so on. For example, nonvolatile memory devices which are represented by a flash memory have been already used in many fields. However, it is said that miniaturization of the flash memory has almost reached a limit. As memories which are usable in behalf of the flash memory, various memories such as a phase change random access memory (PCRAM), a MRAM, and a FeRAM have been developed. Among them, a ReRAM using a material which is adapted to change its resistance value in response to electric pulses applied and retains the states has attracted an attention because of its high compatibility with a standard semiconductor process and high adaptability for miniaturization.
For example, a cross-point ReRAM which is aimed at miniaturization and achievement of a larger capacity is disclosed (e.g., see Patent document 1). In this ReRAM, stripe-shaped lower electrodes are formed on a substrate and an active layer is formed to cover the entire surface. As the active layer, a resistance variable layer which changes its resistance reversibly in response to electric pulses is used. On the active layer, stripe-shaped upper electrodes are formed to cross the lower electrodes perpendicularly to the lower electrodes, respectively. A region where the lower electrode crosses the upper electrode with the active layer sandwiched between them is a memory section. Each lower electrode serves as either a word line or a bit line, and each upper electrode serves as either the bit line or the word line. It is disclosed that such a cross-point configuration can achieve a larger capacity.
In the cross-point ReRAM, a diode is incorporated to be arranged in series with a resistance variable layer to avoid an influence of resistance variable layers belonging to other rows or columns, which occurs when reading a resistance value of the resistance variable layer formed at a cross point.
For example, a ReRAM is disclosed, comprising a substrate including two or more bit lines arranged in parallel with each other, two or more word lines which are arranged in parallel with each other so as to cross the bit lines, respectively, resistive structures formed on the bit lines at positions where the bit lines cross the word lines, respectively, and diode structures formed on the resistive structures in contact with the resistive structures and the word lines, lower electrodes formed on the substrate, resistive structures formed on the lower electrodes, diode structures formed on the resistive structures, and upper electrodes formed on the diode structures (see Patent document 2).
In such a configuration, a unit cell structure can be formed to have a stacked structure including a single diode structure and a single resistive structure which are stacked together continuously, and an array cell structure is easily attained.
As an example of sharing wires, a cross-point MRAM is disclosed, in which a word line is shared and diode elements and MTJ elements are symmetrically formed on and below the wire, respectively (e.g., see patent document 3). In this configuration, platinum is used for the wire and silicon is provided on and below the wire, thereby a Schottky diode is fabricated.    Patent document 1: Japanese Laid-Open Patent Application Publication No. 2003-68984    Patent document 2: Japanese Laid-Open Patent Application Publication No. 2006-140489    Patent document 3: U.S. Pat. No. 6,879,508 Specification